In the mechanical seal which is an example of a sliding component, in order to maintain sealing performance in the long term, conflicting conditions of “sealing” and “lubrication” are required to be balanced. Particularly, in recent years, for environmental measures, the demand for reducing friction is further increasing in order to reduce mechanical loss while preventing leakage of the sealed fluid. As the technique for reducing friction, it can be achieved by sliding the faces in a state that dynamic pressure is generated between the sealing faces by means of rotation and a liquid film is interposed therebetween, that is, by creating the so-called fluid lubrication state. However, in this case, positive pressure generates between the sealing faces, so the fluid flows out of the sealing faces from a positive pressure portion. This is the so-called side leakage in bearings and corresponds to leakage in the case of seals.
In liquid seals, liquid has a higher viscosity than gas, so a dynamic pressure effect is obtained due to fine waviness and roughness irregularity of the faces and the like even if they are plain faces. Therefore, a structure giving priority to sealing performance is often adopted. On the other hand, in order to balance “sealing” and “lubrication”, several mechanisms having a pumping effect of pulling the leaked liquid back to the high-pressure side are devised. For example, Patent Citation 1 discloses an invention in which a plurality of spiral grooves for transferring a fluid to a high-pressure chamber side are provided in the circumferential direction on a sealing face of a rotating ring (hereinafter, referred to as “Background Art 1”).
Moreover, as an invention relating to a sliding component, the present applicant earlier applied for an invention in which, as shown in FIG. 6, a plurality of dimples 50 are provided on a sealing face S, a cavitation formation region 50a on the upstream side of each dimple 50 is disposed closer to a low-pressure fluid side and a positive pressure generation region 50b on the downstream side is disposed closer to a high-pressure fluid side, the fluid is sucked in the cavitation formation region 50a on the upstream side, and the sucked fluid is returned from the positive pressure generation region 50b on the downstream side to the high-pressure side (hereinafter, referred to as “Background Art 2”. See Patent Citation 2.).
However, in the above Prior Art 1, if there is a pressure difference between the inner and outer peripheries of the sealing face, a pumping action against pressure is required, and it may be impossible to push back the fluid depending on the magnitude of pressure. Therefore, there was a problem in that, although it is possible to prevent leakage if the pressure difference is small, the leakage amount must be increased if the pressure difference is large.
Moreover, although the above Prior Art 2 is a breakthrough invention in terms of exhibiting both functions of leak prevention and lubrication regardless of the magnitude of the pressure difference of the inner and outer peripheries of the sealing face, the basic shape of each dimple 50 is a crank shape, thereby slightly lacking smoothness in fluid movement from the cavitation formation region 50a on the upstream side to the positive pressure generation region 50b on the downstream side. Therefore, there was a problem in that the generation of dynamic pressure in the low-pressure fluid side X of the positive pressure generation region 50b is excessive, and the distance from the pressure peak position of the dynamic pressure generation region to the low-pressure fluid side cannot be formed so large, which might lead to leakage.